In the continuous casting of metal, particularly steel, a stream of molten metal is typically transferred via a refractory pour tube from a first metallurgical vessel into a second metallurgical vessel or mold. Such tubes are commonly referred to as nozzles or shrouds and possess a bore adapted to transfer molten metal. Pour tubes include submerged-entry nozzles (SEN) or submerged-entry shrouds (SES), which discharge molten metal below the liquid surface of a receiving vessel or mold.
Liquid metal is discharged from the downstream end of the bore through one or more outlet ports. One important function of a pour tube is to discharge the molten metal in a smooth and steady manner without interruption or disruption. A smooth, steady discharge facilitates processing and can improve the quality of the finished product. A second important function of a pour tube is to establish proper dynamic conditions within the liquid metal in the receiving vessel or mold in order to facilitate further processing. Producing proper dynamic conditions may require the pour tube to possess a plurality of exit ports that are arranged so as to cause the stream of molten metal to be turned in one or more directions upon discharge from the tube.
It may be desirable, for a number of reasons, to induce rotational flow within the mold into which the molten metal is being discharged. Rotation of the flow increases the residence time inside the mold liquid pool to enhance the flotation of inclusions. Rotation of the flow also produces temperature homogenization, and reduces the growth of dendrites along the steel solidifying front. Rotation of the flow also reduces the mixing of steel grades when consecutive grades of steel flow through the pour tube without interruption.
Various technologies have been used in attempts to provide rotation of the flow. Electromagnetic stirring devices may be placed below the entry nozzle. Entry nozzles have been designed that can be rotated in use. Entry nozzles have also been designed with curved exit ports tangent to the bore of the tube.
Various disadvantages are seen in the prior art technology. Electromagnetic stirring devices have a limited life in a hostile environment, rotation of entry nozzle permits oxygen to come in contact with molten metal stream, and curved exit ports are not successful in inducing rotational flow in all mold configurations.
A need persists for a refractory pour tube that produces rotational flow in a variety of mold configurations without the use of additional electromechanical devices. Ideally, such a tube would also improve the flow of molten metal into a casting mold and improve the properties of the cast metal.